Piston engines have a plurality of cylinders respectively in which a piston is mounted in a stroke adjustable manner. Each cylinder has at least one inlet valve and at least one discharge valve for charge-changing processes. Modern engines may have a plurality of inlet valves or a plurality of discharge valves.
In order to increase performance, a piston engine can be equipped with an exhaust gas turbocharger with which the pressure in the fresh gas can be increased. In such charged engines, the problem frequently arises, particularly with low rotational speeds, that upon a load step change from partial load to full load, the turbocharger responds in a delayed fashion only owing to the mass inertia of the rotor assembly, which in turn leads to the so-called “turbocharger lag”.
The present invention addresses the problem of providing for a piston engine of the previously mentioned type an improved embodiment that is particularly characterised by a relatively rapid response of an optionally present exhaust gas turbocharger being able to be achieved therewith.
This problem is solved according to the invention by the subject matter of the independent claim. Advantageous embodiments are the subject matter of the dependent claims.
The invention is based on the general concept of opening the exhaust-gas side discharge of at least one of the cylinders earlier than customary, that is to say at a point in time at which the exhaust gas in the cylinder still exhibits a greater pressure as well as a higher temperature compared to that of an opening occurring at a later point in time. Alternatively, the invention is based on the general concept of opening the exhaust-gas side discharge of at least one of the cylinders later than customary, that is to say at a point in time at which the exhaust gas in the cylinder already exhibits anew an increased pressure as well as an increased temperature in contrast to that of an opening occurring at an earlier point in time. By means of the respective measures, the pressure and the temperature in the exhaust gas and thus the enthalpy in the exhaust gas can be temporarily increased after the respective cylinder. In so far as a turbine of an exhaust gas turbocharger is arranged in the exhaust gas line, increased energy for accelerating the rotor assembly is thus available therefor compared to conventional engine timing. The exhaust gas turbocharger can thereby accelerate more rapidly and generate the desired increase in pressure on the fresh-gas side.
This is achieved with the piston engine according to the invention by means of a double overhead camshaft by means of which the discharge valves of the cylinders are controlled. The double overhead camshaft has an inner shaft and an outer shaft coaxial thereto that is configured as a hollow shaft, the two shafts being rotatably adjustable relative to one another. The shafts respectively support cams arranged thereon in a rotationally-fixed manner for controlling the discharge valves. At least one of the cylinders has two discharge valves of which the one can be controlled by means of one cam of the inner shaft, while the other can be controlled by means of one cam of the outer shaft. By changing the relative position, that is to say the relative rotational position between both of the shafts, the discharge time frame can be varied. In particular, the opening time and the closing time can be shifted independently of one another to early and/or late. In order to increase the enthalpy content of the expelled exhaust gas, it is thus possible in particular to shift the opening time of the discharge time frame to be earlier or later, in that the relative position between the shafts is correspondingly varied. The closing time of the discharge time frame can remain constant.